refactor(tenant): drop legacy plugin_installations, migrate monitoring to desktop_clients

Hard cutover from the browser-extension plugin flow to desktop clients:
remove plugin_installations/plugin_sessions tables and related service,
handler, router, and generated model code; migrate monitoring quotas
and collector types to desktop_clients (UUID primary_client_id);
recreate platform_access_snapshots keyed by client_id; update dev-seed
and callback types accordingly; mark legacy design docs as historical.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/migrations/20260410103000_create_monitoring_tables.up.sql

@@ -1,11 +1,11 @@
 CREATE TABLE tenant_monitoring_quotas (
     tenant_id                BIGINT PRIMARY KEY REFERENCES tenants(id),
     max_brands               INT NOT NULL DEFAULT 1,
-    collection_mode          VARCHAR(20) NOT NULL DEFAULT 'plugin',
+    collection_mode          VARCHAR(20) NOT NULL DEFAULT 'desktop',
     question_selection_mode  VARCHAR(20) NOT NULL DEFAULT 'best_effort_all',
     collect_frequency        VARCHAR(20) NOT NULL DEFAULT 'daily',
     enabled_platforms        JSONB NOT NULL DEFAULT '["deepseek","qwen","doubao"]'::jsonb,
-    primary_installation_id  BIGINT REFERENCES plugin_installations(id),
+    primary_client_id        UUID,
     task_daily_hard_cap      INT NOT NULL DEFAULT 36,
     plan_tier                VARCHAR(20) NOT NULL DEFAULT 'free',
     created_at               TIMESTAMPTZ NOT NULL DEFAULT NOW(),